The present invention relates to flow control systems, and more particularly, to such systems which are utilized to provide fluid to a vehicle steering cylinder or steering motor.
It will be apparent to those skilled in the art that the present invention may be utilized in any system which controls the flow of fluid from a variable flow and pressure fluid delivery source to a fluid actuated device. However, it is especially adapted for use in the hydraulic control circuit of a vehicular hydrostatic steering system, and will be described in connection therewith.
In a typical hydrostatic steering control system of the type in which the present invention may be used, there is a steering control valve disposed in series flow relationship between the variable fluid source (such as the pump or pump & priority valve), and the steering motor (generally a piston and cylinder). The steering control valve defines a variable orifice which controls both the amount and direction of fluid flow through the steering control valve, and therefore, the rate and direction of actuation of the steering cylinder. The variable orifice in the steering control valve varies from a minimum orifice area to a maximum orifice area in response to the turning of the steering wheel, and as the orifice area increases, the flow through the orifice increases. A particular steering control system is designed to operate at a predetermined rate of fluid flow through the steering control valve, and at a predetermined maximum valve deflection (for example, 10.degree. of rotation of the valving). As is well-known in the art, the predetermined flow rate will occur at the maximum valve deflection only in response to a particular pressure drop across the orifice, and if the pressure drop across the variable orifice is greater than that for which the steering control valve was designed, the predetermined flow rate will occur at a smaller valve deflection (for example, 5.degree. of rotation). Achieving the maximum rate of actuation contemplated by the design of the steering control valve, but at a smaller valve deflection, results in an undesirable increase in the sensitivity of the steering system.
It frequently occurs that the same basic steering control system may be utilized with any one of a number of different pump arrangements, each having a different standby pressure capability, and therefore, each delivering a substantially different fluid pressure to the inlet of the steering control valve. It has been common practice in conventional steering control systems to maintain the desired relationship between valve deflection and the steering control valve and flow rate through the valve by changing the configuration of the variable orifice (i.e., change the area of the orifice at a given valve deflection) to satisfy each of the different pressure drops imposed on the steering control valve. Such modifications of the steering control valve for different fluid sources has added undesirable and otherwise unproductive engineering and manufacturing cost to such steering control systems and restricted the flexibility in use of such systems.